The present invention generally relates to the conversion of low dynamic range images to high dynamic range images.
Selected digital photograph imaging devices and digital video imaging devices can capture relatively high dynamic range images. High dynamic range images capture most of the dynamic range of real world luminance which are more readily displayed with a display having a corresponding high dynamic range. Accordingly, capturing images with a suitably high dynamic range together with rending the captured images with a suitably high dynamic range display provides a representation of the image content that is generally consistent with real world illumination ranges.
Most of the image capture devices only have the capability of capturing light on the order of about three orders of magnitude as opposed to generally twelve orders of magnitude of real world scenes observable by the human visual system. Traditionally, displays represent a digital image with a set of 256 values per color channel, with a maximum of 65,536 different values. In general, the limitations of only a set of 256 values per color channel may be referred to as low dynamic range images and low dynamic range displays.
As higher dynamic range displays are more readily available, there is an increasing demand to display the lower dynamic range images on the higher dynamic range displays. One technique to convert lower dynamic range images to higher dynamic range images is generally referred to as reverse tone mapping. Reverse tone mapping may be generally performed in two stages. The first stage is performed to inverse map the luminance of a lower resolution input image into an expanded high dynamic range luminance image. As a result of image quantization, this results in a loss of details and introduces noise in high luminance regions of the image. The second stage remediates the results of image quantization by smoothing such regions while also allowing for potentially increasing the dynamic range of the image content.
Another technique to convert lower dynamic range images to higher dynamic range images is to linearly scale the image data. Unfortunately, linearly scaling the image data fails to capture the tonescale aspects of specular highlights in the images.
Another technique to convert lower dynamic range images to higher dynamic range images is to use multiple lower dynamic range images with multiple exposures to determine higher dynamic range images. However, typically there are not multiple different exposures of the same scene image.